Wednesday, September 14, 2016

Too Far Planets

Suppose we consider as a thought example a planet which was the first to develop intelligent life which ran out of resouces and wanted to preserve its civilization on a planet similar to its own. This type of planet is what we call alpha-habitable, and means a planet with animals and oxygen and soil and plants, one which is just great for colonization and one which is what all science fiction writers envision when they write those great stories about interstellar voyaging.

Unfortunately, it seems they are not all that common in the galaxy. Suppose in our example there is just one alpha-habitable planet in the whole galaxy at the time the first alien civilization starts searching. It is likely on the galactic disk, and an average distance might be thirty-five thousand light years. Suppose the alien civilization is highly motivated to get to this planet. How might they attempt it?

When we on Earth scan the skies for future Earths, we do not look out this distance. Instead, we look out a hundred light years or so. If we think about going there at a fractional iight speed, one percent or ten percent come to mind. This means a trip of a thousand to ten thousand years. An immense time for us, as we only have recorded civilization for a few millennia. If you ask an engineer about constructing something which would last longer than the Roman Empire, in space, with no failures significant enough to doom the voyage, you should expect ridicule. We simply have no experience with this length of time. Stone monuments might last this long, but nothing with moving parts.

An alien civilization, such as in our example, might last a million years before it needed to colonize a new solar system. If you asked an engineer in that civilization about something that had to last a thousand years without significant failures, he might point out some examples. He might refer to textbooks written five hundred thousand years before about building in reliability. This type of trip doesn't appear as ridiculous to the alien civilization's engineer as it does to ours. So, if they were lucky enough to have an alpha-habitable planet located only a hundred light years away, they would certainly think they could get there.

But a planet thirty five thousand light years is another challenge. At the range of speeds likely, this means three hundred and fifty thousand years up to three and a half million years. Now that the travel time approaches or exceeds the maximum duration of the civilization, the question has shifted from the eminently possible to the outlandish and absurd.

Reliability is not the only factor that seems impossible. What about providing enough energy to supply the ship for the length of the trip? There would be no propulsion power needed, as the ship would use the initial part of its propellant coming up to speed and preserve the rest until it was necessary to decelerate upon the approach to the destination planet. It is the hotel load, the power needed to keep the systems in the ship running for that length of time. Perhaps all that is needed is a clock with a switch to turn the systems of the ship back on once the neighborhood of the destination was reached.

What powers the clock? What source of energy can survive tens or hundreds of thousands of years of being radiated in the interstellar void? Nothing chemical would work, as these systems have a shelf life and shelf life is not denominated in units of this size. A nuclear reactor, a bit overpowering for a clock, would suffer from decay as well as the effects of the interstellar radiation. A bank of fuel cells would suffer from erosion for the electrodes, as well as ablation of other components. A radioactive source is going to be irradiated as well, even if something with a very long lifetime was chosen and properly sized.

Robotic solutions simply don't work and so the inevitable conclusion is that something biological is needed. Biological systems are self-repairing, and seem to offer the option of having a reliability higher than robotic systems, but with quite different requirements and design. This alternative might solve the reliability problem, but it is for us a complete unknown.

The energy problem might be solved if there were intermediate stops where energy sources could be tapped. This prevents extreme problems. To have a one-stop trip, instead of a non-stop, the propellant to slow down and stop the ship has to be carried, and also the propellant to re-accelerate the ship. And we all know about the accumulation of mass aboard a rocket ship. Mass does not go up linearly, but because it is necessary to initially accelerate all the fuel on board, even more is needed. Many times the original fuel is needed for a one-stop trip. For a multi-stop trip, the numbers become insane.

What about picking up fuel at each stop? If this was possible, it would relieve the ship of the necessity of carrying multiple times as much fuel as minimally necessary for a non-stop trip. Still, the energy necessary to power that propellant, even if it were scooped up at each stop, is necessary. A huge amount of energy required becomes even more huge.

What about finding stops where the energy needed could be obtained? There are no fueling stations on random planets in the galaxy. Somehow the materials needed to re-constitute the ship’s fuel would have to be gathered, and the various pieces of equipment necessary to turn these materials into fuel have to be carried and maintained for the entire trip, except for the portion from the last fueling stop to the final destination. If something like antimatter was used to keep the weight of the energy system down as low as possible, now, with a fueling stop it is necessary to carry all the equipment needed to convert whatever energy source is available on some planet back into antimatter. Since conversion is not likely to be anything like 100% efficient, even more fuel materials must be collected.

It seems that going this distance is simply beyond the bounds of possibilities. Some planets are simply too far out for even an aspiring civilization to attempt. If, as in our example, there is only one planet and it is that far, they simply must descend to sustainable life, or perhaps make the decision to no longer continue their civilization.

Perhaps a guess toward how far out too far out is would be a thousand light years. A hundred light years would be doable for an advanced alien civilization in possession of sufficient resources and somewhat beyond that as well. This has implications for how fast the galaxy would become occupied, and how fast the existing number of alpha-habitable planets would be exhausted.

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